1. Technical Field of the Invention
This invention relates generally to the use of signal processing techniques for determining the performance of cabling connections, and, more particularly, to the development and use of time domain limits to determine the location and source of faults in cabling systems.
2. Background of the Invention
The transmission performance characteristics of modem high speed data communication twisted pair cabling systems are defined by various international and industry working bodies (standards organizations) to assure standard data communication protocols can successfully be transmitted across the transmission media. These data communication cabling systems (known as links) typically consist of connectors (modular 8 plugs and jacks) and some form of twisted pair cabling. The requirements for important RF transmission performance parameters such as, among others, Near End Crosstalk (NEXT), Return Loss, Insertion Loss, and Equal Level Far End Crosstalk (ELFEXT) are specified as a function of frequency. To assure compliance of cabling systems with these requirements, various field test instruments are available to certify that installed cabling meets the required frequency domain limits. These instruments perform various measurements to verify compliance with the standards and provide an overall Pass or Fail indication of the link.
When failures are detected in a link, a trouble-shooting process must be done to make the link compliant with the requirement. However, currently known field test instruments generally do not provide simplified diagnostic information to help locate and determine the reason for failures. Determining the cause of some RF transmission parameter faults can be difficult since the overall link performance often depends on the performance of the individual components and installation techniques of the link. Until now, there have been few simple yet accurate methods to determine if a fault is at the connection or the within the cable itself. That is, the frequency domain data that is processed by currently known field test instruments does not provide readily interpretable information about the cause of the failure.
In the past, trouble-shooting of NEXT and Return Loss faults with normal field test equipment has generally been done on a trial and error basis as it typically requires skill levels normally not available to the cable installation industry. Often, misinterpretations of frequency domain data are made, resulting in substantial unnecessary rework of the link.
For example, transmission parameter requirements have been established for various classes or categories of performance for structured data communication wiring systems. There are current standards for Category 3, (10 Megabit/second data systems), Category 5 (100 Megabit/second data systems) and new emerging standards (Category 6 and Category 7) to support even higher data rate systems. The Category 5 cabling system is a mature technology and few installation problems exist due to the excess margin that has evolved in the individual component designs.
However, with the emergence of higher performance Category 6 and Category 7 cabling systems, a significant percentage of such links do not meet desired performance levels. These links thus require a fault diagnosis. In general, the failures in these links are due to a lack of transmission performance margin in individual components and higher degrees of sensitivity to the installation practices that have been used for Category 5 and other systems that are pervasive in the market.
For example, a typical link 100 in a structured cabling system and associated field test configuration is shown in the FIG. 1. Link 100 consists of a data communication patch panel 110 (for example, located in a wiring closet), four-pair twisted pair cable 120, and a data connector 130 in a work area. Field testing of the link transmission performance is typically done with field test equipment 140 that runs a suite of frequency domain tests from both ends 122 of link 100. The field test equipment is interfaced through short test lead cables 150 that connect to data communication jacks 110, 130 of the link under test. Tests of NEXT, Return Loss, Insertion Loss, ELFEXT and the like, are typical measurements performed by these instruments to certify transmission parameters. The measurements are then compared to a set of known limit criteria established for specific categories of performance. A Pass/Fail indication is then made.
An example of a NEXT measurement and the performance limit for a Category 6 link 100 is shown FIG. 2. The measured performance of link 100 exceeds the limit at one or more measured frequency points. The link is considered to have failed because it does not meet desired performance standards. The data in FIG. 2 shows a failure was detected at several regions of the frequency spectrum. A challenge in diagnosing this failure is determining if the cause of the failure is the connectors 110,130, cable 120, or the installation practices employed to terminate cable 120 to connectors 110,130. There is little information in the frequency domain graph of the magnitude of NEXT to help with the problem isolation process. Thus, a significant first step in the diagnostic process for the example shown in FIG. 2 is to locate the position of major contributors of NEXT in link 100 in time, and hence, distance.
Those skilled in the art understand the conversion from the frequency domain to the time domain may be accomplished by applying an Inverse Fourier Transform process to the magnitude and phase NEXT frequency domain data. The result of this conversion provides a plot of changes in NEXT vs. time/distance. For example, the NEXT time response for the preceding example is shown in FIG. 3. As seen in the graph, there are a number of large sources of NEXT. The first major source is connector 110 located approximately two meters from the xe2x80x98nearxe2x80x99 end 110 of test cable 120. As is apparent from the graph shown in FIG. 3, other large source NEXT exist within cable 120 itself
Time domain techniques have been used to identify sources of NEXT in current field test equipment. However, knowledge of NEXT vs. time information does not necessarily aid in diagnosing the reason for failure. Time data itself can be useful since it identifies sources of NEXT as a function of distance; but this data itself does not provide information as to whether connector 110 or cable 120 performance is within required performance ranges. In FIG. 3, conventional wisdom points to connector 110 as the non-compliant component since it is the largest source of NEXT. However, without additional data, there is no definite information as to how to resolve the failure.
One method of trouble-shooting the NEXT failure in link 100 is to disassemble link 100 and qualify the NEXT characteristic of each component relative to the component requirements. FIG. 4 shows NEXT measurement results for both cable 120 and connector 110 compared to one another and the respective NEXT limit for each component. In this case, connector 110, which was the highest NEXT source, falls within acceptable NEXT limits, and thus its performance requirements. However, the graph shows NEXT in cable 120 exceeds acceptable NEXT limits for cable 120. The cause of this failure is cable 120 and not connector 110. Time constraints, the knowledge and experience of cable technicians, and the impracticalities of reworking cabling systems make such disassembly for diagnosis impractical to do in the field.
Further, as FIG. 4 shows, the field diagnosis problem is quite difficult. Frequency domain measurements of link 100 do not generally provide fault location information. While time domain techniques are useful for locating sources of NEXT, they lack limit information to determine the components that are non-compliant. Thus, combining the use of time domain measurements with a method to convert the frequency domain component limits to the time domain would produce a major enhancement to the field diagnostic capability.
Another possible method to calculate the time limit and display time domain NEXT and Return Loss data is to attempt to normalize the limit data for attenuation with distance. Generally, this method generates a flat time limit line and produces data that has a flatter response with distance. Such a normalization process is disclosed in U.S. Pat. No. 5,698,985, entitled xe2x80x9cCrossTalk Measurement Instrument with Source Indication as a Function of Distance,xe2x80x9d issued to Bottman and assigned to Fluke on Dec. 16, 1997 (the xe2x80x9c""985 patentxe2x80x9d). While this approach appears simple and attractive, in practice it is difficult to properly implement and can provide misleading information. This is because the attenuation due to cable 120 is both a function of frequency and distance. Generally, the attenuation increases at approximately the square of the frequency. To normalize the time response for length generally requires special processing with time dependent filters that account for the length and transfer function. Simple scale factor normalization based on length as described in the ""985 patent tends to enhance the low frequency contribution of the data with increasing length, leading to possible misdiagnosis of fault conditions.
Thus far, the examples and discussion have been related to NEXT measurements. However, additionally, return loss measurements have many of the same diagnostic issues that can be addressed in a similar manner using time domain limits and processing techniques.
Return loss measurements provide a measure of the ratio of the reflected energy to the transmitted energy. Generally, signal reflections occur within data communication cabling due to impedance changes in the transmission media. Major sources of reflection can occur at connections due to connecting hardware, poor installation, a change in cable impedance and the like. The normal certification tests are done in the frequency domain to verify the parameter is compliant with the requirement. The frequency domain Return Loss measurement provides an overall measure of reflected energy. However, the measurement does not separate the reflected signals from each of the individual components.
Accordingly, a method for determining if the transmission parameter fault is at the connection or in the cable would be of benefit to the cable installation industry. Since typical links are constructed of a single connection at each end of the cable, isolating the problem to either the connection or the cable allows for rapid rectification of the problem. Further, the application of time domain techniques along with time limits for the reflection that is allowed at a connection point would provide the installer with a means to diagnose Return Loss faults. Enhanced diagnostic capabilities are required to help successfully install and certify new higher performance cabling systems to significantly improve the productivity of the installation process with such diagnostics.
The present invention provides methods for using time domain analysis of NEXT, Return Loss and the like, in conjunction with the application of time or distance referenced limits to verify and determine compliance of the performance requirements of connections in a typical link. Time domain analysis of NEXT and Return Loss data. suitably provides the performance characteristics of a link as a function of time or distance. When coupled with time or distance performance curves for connections, it can be determined if the transmission fault is at a connection, in the cable, or in another component of the link. The time limit curves for connections can be generated based on the frequency domain performance requirements for connecting hardware for a specific level of performance. The connection time limit curves thus provide an interpretation means to determine if the connection is within performance standards, affecting improved isolation of the fault condition.